1. Technical Field
The present invention relates to a printing technology for printing an image by using dots.
2. Background Technology
A technology for reproducing multi-gradation images is used in a printing device such as a printer in which one or more types of dots are recorded on a printing medium. In recent years, multi-gradation technologies have been dramatically developed so that it is possible to produce so-called photographic quality images by combining two small and large sizes of multi-colored dots such as cyan (C), magenta (M), yellow (Y), and black (K), and controlling the distribution of these dots. For the arrangement of these dots, there are a dot concentration form, which concentrates dots as a typical halftone dot type, and a dot dispersion form, which disperses dots as many as possible in the arrangement. In a case of the dot dispersion form, because of advances in the technology for analyzing a distribution of dots in a spatial frequency range, it is well known that an image quality can now be improved by maintaining in the dot distribution noise characteristics where the number of components at or below a predetermined frequency is kept as low as possible in a spatial frequency range.
These noise characteristics are typically blue noise characteristics. The blue noise refers, for example, to characteristics in which the spatial frequencies of the image formed with dots uniformly to reproduce an image having a constant gradation value include substantially no components at or below a predetermined frequency. While human eyes are sensitive to the low-frequency components below a certain level, but the high-frequency components are not very visible. Thus, images with these blue noise characteristics have a smooth, high-quality feel. A well-known image formation technology having the blue noise characteristics has been disclosed in Patent Document 1.
U.S. Pat. No. 5,341,228 (Patent Document 1) is an example of the related art.
In one of the methods for forming dots on a printing medium, serial printers are used to form dots in the forward and reverse actions of the print head in a width direction of the printing medium (hereinafter, the head movement in this direction calls a main scanning). Among these serial printers, an inkjet printer performs forming dots by discharging ink droplets onto the printing medium from a nozzle. For discharging the ink droplets, it is well known as a method of using a deformation of a piezo-element caused by increasing voltage or a method of using foam (bubbles) generated by heating the ink, or the like. With any of the methods, it is necessary the plurality of main scanning of the print head to form images by the dots. With that, to achieve an improvement of the printing speed, the technology has been developed as a bi-directional printing (as called BI-D printing) to form an image with the combination of dots formed by the reverse action of the print head and the forward action of the print head. Also, to achieve an improvement of the formed image quality, the technology has been developed as a multipath printing to complete each raster by the plurality of main scanning of the print head.
In the image forming technology of the plurality of main scanning by the print head, it causes the reduced image quality due to the shift between a formation position of the dots formed by the first main scanning and a formation position of the dots formed by another main scanning. There are the printing devices to establish an appropriate formation position of dots so that a particular position of the dots is formed without shifting. However, the problem of the shift appears significantly in these printing devices. In a printer which performs printing by the dot formation in the dot concentration form, FIG. 28 is an explanatory drawing showing one example of the dot arrangements in a case where the dot formation positions are shifted in the forward action and the reverse action. In FIG. 28, the symbol “◯” is shown as a dot formation position formed at the forward action time and the symbol “●” is shown as a dot formation position formed at the reverse action time. FIG. 28A shows in a case where the dot formation position is completely adjusted so that the dot formation positions at the forward action time and the reverse action time are not shifted (hereinafter, it calls “shift 0 in the positional relationship”). The size of dots in FIG. 28 is the smallest true circle which totally covers each pixel assumed as rectangle accordance with horizontal and vertical resolutions.
FIG. 28B shows dots forming aspect in a case where the dot formation positions at the forward action time and the reverse action time were shifted by 1 pixel in the main scanning direction from shift 0 in the positional relationship. In the similar manner, FIG. 28C shows in a case of the dot formation position shifted by 2 pixels in the main scanning direction. FIG. 28D shows in a case of the dot formation position shifted by 3 pixels in the main scanning direction. FIG. 28E shows in a case of the dot formation position shifted by 4 pixels in the main scanning direction. As shown in the drawings, as the dot formation positions in the main scanning direction at the forward action time and the reverse action time become deviating from shift 0 in the positional relationship, the dot arrangement in the dot concentration form was destroyed and the ratio of that each dot independently covers one pixel was increased. Also, in this case, even though the same numbers of dots were arranged, it understood that the ratio of that dots cover the surface of the printing medium was increased (hereinafter, it calls “coverage”). In a case of changing the dot coverage, it changes the brightness of images (brightness, density, reflectance or the like) or the color tone (color phase or colorfulness). In particular, FIG. 28 shows an example of when dots are alternately formed in each raster at the forward action time and the reverse action time. However, when dots are formed in a column arrangement at the forward action time and the reverse action time or when dots are formed in a crossed arrangement (checker board design), it does not change the occurrence of the change in the brightness or the color tone of the images.
On the other hand, there is a printer using the dot arrangement in the dot dispersion form. For example, in the printer which performs printing dots including the blue noise characteristics as discussed above, when forming an image with the lower gradation values, the dots are preferably not close to each other in the arrangement. However, as discussed above, in a case where the dot forming position during the forward action and the reverse action are shifted from the shift 0 position, the arrangement of the dots are deviated from the preferred arrangement. In general, in the dot arrangement in the dot dispersion form, when the dot formation position is shifted, the overlapping dots are increased so that in contrast with the dot concentration form, the coverage in the dot dispersion form is reduced. In this case, the change in the brightness or the color tone of the images can occur as a result of the coverage variation.